This invention relates to optical discriminators. Among the uses to which such discriminators may be put, attention is particularly drawn to those described in GB 2 107 147 A in which amplitude modulation of the optical output of an injection laser is provided by modulating that laser so as to induce frequency modulation with relatively small accompanying amplitude modulation depth, and this output is then passed through the discriminator to convert the frequency modulation to amplitude modulation with an enhanced modulation depth. This manner of achieving amplitude modulation with a substantial modulation depth can be advantageous in providing a signal of reduced spectral width compares with that produced by direct amplitude modulation of the injection laser output, and this is potentially useful in high bandwidth long distance transmission systems in which bandwidth or distance is achieved by dispersion effects.
Optical discrimination can be provided by the classical form of Mach Zehnder interferometer in which a first partial reflector divides an input signal equally into two beams which propagate for slightly different distances before being optically interfered at a second partial reflector. Interference at the second partial reflector is only complete if the two interfering beams have identical states of polarisation (SOP's). In the classical form of Mach Zehnder interferometer the light propagates in free space in its two limbs that extend from the first partial reflector to the second, and so the interfering beams do have matching SOP's. A Mach Zehnder interferometer can also be formed in optical fibre format by connecting a pair of 2.times.2 optical fibre 3 dB couplers optically in tandem with the two output ports of the first 3 dB coupler connected by two optical fibre links of unequal length to the two input ports of the second 3 dB coupler. Birefringence can be introduced into circularly symmetric single (degenerate) mode optical fibre by bending it or twisting it, and therefore an optical fibre format of Mach Zehnder may include a certain amount of birefringence in its two limbs. If the birefringence is not the same in the two limbs, the SOP's at the second 3 dB coupler may not be matched, in which case interference is incomplete--and in the worst possible case may have orthogonal SOP's, in which case there is no interference whatever.
When polarised light propagates in a certain direction through birefringent material its SOP does not in general remain constant, but evolves in a manner that is conveniently described with reference to a Poincare sphere as depicted in FIG. 1. On this sphere the points H and V represent horizontally and vertically polarised states, the points L and R represent left-handed and right-handed circularly polarised states, and the points P and Q represent two linearly polarised states with polarisation planes inclined at 45.degree. to the horizontal and vertical planes. Each possible state of elliptically polarised light is similarly represented by some point on the sphere lying between the equatorial great circle through HQVP and the two poles L and R. Thus on the Poincare sphere all orthogonally polarised states are represented by diametrically opposed points on the sphere. For light propagating in a particular direction through a birefringent medium there are two particular orthogonal SOP's, known as eigenstate SOP's, which will propagate without change of SOP. Light launched into the medium in that particular direction with any other SOP will propagate through the medium with its SOP steadily evolving in a manner that describes a circle on the Poincare sphere that passes through the starting point SOP and lies in a plane normal to the eigenaxis that joins the two eigenstate SOP's.
Therefore, if there is birefringence in the two limbs of a Mach Zehnder interferometer, it can still function on its own as an efficient optical discriminator provided that it is arranged that the light is launched so that it propagates in the two limbs with eigenstate SOP's, or if the birefringence is arranged so that the SOP's evolve in the two limbs in a manner providing matching SOP's at the far end.